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Bureau of Mines Information Circular/1985 



Continuous Radiation Working-Level 
Detectors 



By R. F. Drouilard and R. F. Holub 



-^sr^ 




UNITED STATES DEPARTMENT OF THE INTERIOR 



75: 



w 

'If/NES 75TH AV»^ 



Information Circular 9029 



Continuous Radiation Working-Level 
Detectors 



By R. F. Droullard and R. F. Holub 




UNITED STATES DEPARTMENT OF THE INTERIOR 

Donald Paul Model, Secretary 

BUREAU OF MINES 
Robert C. Morton, Director 







Library of Congress Cataloging in Publication Data: 



DrouUard, R. F. (Robert F.) 

Continuous radiation working-level detectors. 

(Information circular ; 9029) 

Bibliography: p. 17-18. 

Supr. of Docs, no.: 1 28.27:9029. 

1. Nuclear counters. 2. Radon— Measurement. 3. Uranium mines 
and mining— Safety measures. 4. Mine gases— Measurement. I. Holub, 
Robert F. II. Title. III. Series: Information circular (United States. 
Bureau of Mines) ; 9029. 



TN295.U4 [TN490.U7] 622s [622.8] 85-600009 



1 


2 


2 


2 


4 


7 


8 


9 


9 


9 


10 


10 


11 


12 


13 


15 


15 


17 


19 



CONTENTS 

Page 

Abstract 

Introduction 

Three CWL detector designs and measurement methods 

Backside of filter counting 

Filter surface counting 

Ambient air counting 

^ - Ai r sys terns 

_- Factors affecting accuracy 

_ Volumetric airflow variations 

Radon daughter mixture variations 

Plateout losses 

A Other sources of error 

^ Calibration 

~ Attachments for model B-lOO CWL detector 

'^ CWL detector applications 

O Commercial CWL instruments 

Conclusions 

References 

Appendix. — Method for measuring efficiency of a beta detector 

ILLUSTRATIONS 

1. Unassembled model B-lOO CWL detector 3 

2. Model B-lOO CWL detector 3 

3. Block diagram of CWL monitoring system 4 

4. Unassembled model FC CWL detector 5 

5 . Model FC CWL detector 5 

'^ 6. Model FC CWL detector with alpha-detecting diode 6 

»-^ 7. Model FC CWL detector with beta shield 7 

*J 8. Model Beta-8 experimental CWL detector 8 

— 9. Block diagram of typical air system for use with a CWL detector 8 

— 10. Inherent uncertainty plot for a gross alpha CWL detector 9 

—r- 11 . Inherent uncertainty plot for a gross beta CWL detector counting from back 

"^ side of filter 10 

^ 12. Inherent uncertainty plot for a gross beta CWL detector counting from col- 
lection side of filter 10 

13. Two model B-lOO CWL detectors equipped with input tubes 12 

14. Effect of input screen on attached-unattached radon daughter activity 12 

15. Effect of input screen on attached-unattached radon daughter activity in 

the Twilight Mine 12 

16. Two model B-lOO CWL detectors equipped with Zeleny tubes 13 

17. Model B-lOO CWL detector with Zeleny tube containing a charged wire 13 

18. Charged particle collection using a negatively charged wire in a Zeleny 

tube on a model B-lOO CWL detector 14 

19. Effect of a ventilation system using a heat exchanger on the working-level 

exposure in a dwelling 14 

20. A commercial CWL monitoring system in operation in a mine 15 

21. A commercial CWL monitoring system undergoing tests 15 

22. Comparison of working-level values measured by alpha and beta CWL 

detectors 16 

23. Commercial version of model B-lOO CWL detector 17 



11 



TABLE 



1. Effect of filter-to-detector distance on count rate, 



Page 
6 





UNIT OF MEASURE ABBREVIATIONS 


USED 


IN 


THIS 


REPORT 


cm^ 


square centimeter 


irnii 






millimeter 


cm-^ 


cubic centimeter 


9 






square millimeter 


cpm 


count per minute 


p,m 






micrometer 


dis/min 


disentegration per minute 


|J,S 






microsecond 


h 


hour 


pCi 


/L 




picocurie per liter 


L 


liter 


pCi 


/(L/WL) 


picocurie per liter 












per working level 


L/min 


liter per minute 


pet 






percent 


MeV 


million electron volts 


V 






volt 


min 


minute 


WL 






working level 


mg 


milligram 


yr 






year 


tag/cm^ 


milligram per square 
centimeter 











CONTINUOUS RADIATION WORKING-LEVEL DETECTORS 

By R. F. Droullard ^ and R. F. Holub^ 



ABSTRACT 

The Bureau of Mines has used gross alpha and gross beta detectors to 
continuously measure radiation working levels for a number of years. 
During this time, improvements have been made in the design and per- 
formance of continuous working-level (CWL) detectors. This report dis- 
cusses the improved designs and some of the operating principles and 
applications of CWL detectors in the measurement of radon daughter 
products in mines and dwellings. 



'Supervisory geophysicist. 
^Research physicist. 
Denver Research Center, Bureau of Mines, Denver, CO. 



INTRODUCTION 



The inhalation of radon daughter prod- 
ucts may result in lung cancer. These 
products can be an especially significant 
problem for underground miners in uranium 
and other types of mines where the daugh- 
ter products of 222Rn ^^^ always present 
in the mine atmosphere. Exposure to the 
ionizing radiation of 222^^ is measured 
in working levels (9^).^ One working lev- 
el denotes any combination of the short- 
lived radon daughter products in 1 L 
of air that will result in the ultimate 
emission of 1.3 x 10^ MeV of alpha 
energy. 

The most common method of measuring 
working-level exposures is to collect 
a grab sample on a filter and measure 
the alpha and/or beta activity. While 
this approach is satisfactory for some 
applications, it did not meet the needs 
of much of the radiation hazard research 
work performed by the Bureau, and so 
the Bureau initiated efforts to develop 
a CWL detector (5). A CWL detector 



differs from other automated samplers in 
that air sampling is continuous, whereas 
other methods use an electromechanical 
grab sampler that samples for predeter- 
mined periods. Radon daughter measure- 
ments using this latter method of sam- 
pling provide information that represents 
conditions at the time the sample is 
collected. Most of the CWL detectors 
discussed in this report have a response 
time that lags behind actual activity 
levels by about 1 h. 

Earlier work by the Bureau on CWL de- 
tectors was reported in 1977 (4^). Since 
that time, improvements have been made in 
CWL detector designs and applications (2, 
6). CWL detectors developed by the Bu- 
reau and industry have been evaluated us- 
ing the Bureau's radiation-hazard test 
facilities (3) , and in operating mines 
and dwellings. This report provides up- 
dated information on the use of CWL de- 
tectors for measuring radon daughter ac- 
tivity over extended time periods. 



THREE CWL DETECTOR DESIGNS AND MEASUREMENT METHODS 



All but one of the CWL detectors dis- 
cussed in this report measure the gross 
radon daughter activity collected on a 
filter by means of a vacuum pump. The 
type of radioactivity measured depends on 
the type of detector used. In any case, 
the detector output, in the form of 
pulses, is counted, and, through calibra- 
tion, each count is assigned an equiva- 
lent amount of alpha energy, which is 
expressed in working levels. Three CWL 
detector designs developed by the Bureau 
are discussed in this section. 

BACKSIDE OF FILTER COUNTING 

Figure 1 shows an unassembled model 
B-lOO CWL detector. Figure 2 shows an 
assembled detector mounted on a metal box 
that contains a high-volpage power supply 
for the Geiger-Mueller (GM) tube and an 
electronics system for signal processing. 

^Underlined numbers in parentheses re- 
fer to items in the list of references 
preceding the appendix. 



In the model B-lOO CWL detector, the 
beta activity of the radon daughters col- 
lected on the filter is measured by the 
GM tube from the back side of the filter 
collection surface. The primary reason 
for this method of measuring the activ- 
ity is to provide an unobstructed path- 
way to the surface of the filter. This 
reduces problems with radon daughter 
plateout (removal of radon daughters from 
the air by their attachment to surfaces) 
and makes attachment of accessories less 
difficult. (Attachments for the B-lOO 
detector are discussed in a later 
section. ) 

The B-lOO detector uses a type-7311 GM 
tube with a diameter of 5.36 cm and a 
thickness of 1.53 cm. The tube has a 
mica window with an areal density of 
about 2 mg/cm^. An aluminum foil is 
placed over the GM-tube window to prevent 
detection of alpha particles. The foil 
also helps to protect the interior of the 
GM-tube housing from moisture. The de- 
tector is operated with an anode resist- 
ance specified by the manufacturer and a 





o 



mi-^ 



mmmtimsmmmf'w: 



FIGURE 1. - Unassembled model B-100 CWL detector. From left to right: back cover plate with signal and 
air connectors, housing for Geiger-Mue 1 ler (CM) tube with CM tube clamp and O-ring seals, pancake-type GM 
tube, filter backup screen, glass-fiber fi Iter, rubber fi Iter gasket, and fi Iter c lamping plate w ith reto ining screws. 




FIGURE 2. - Model B-lOO CWL detector mounted on metal enclosure for associated electronics, with beta 
shield. 



bias voltage determined by measuring the 
tube's operating plateau. 

The signal-processing electronics con- 
sist of an amplifier, discriminator, 
dead-time control, and line driver. The 
system dead time is around 50 |is. A 
block diagram of a CWL monitoring system 
used by the Bureau is shown in figure 3, 



ll5Vac 



In some applications of the B-lOO CWL 
detector, beta particles from the decay 
of airborne 214gj (RaC) have sufficient 
energy to reach the GM tube through the 
filter and its backup screen. These un- 
wanted particles can be eliminated by 
using a beta shield as shown in figure 2. 
The shield is spaced about 2 cm from the 
detector housing to avoid losses of radon 
daughters through plateout on the shield. 

FILTER SURFACE COUNTING 



115 Vac 




Aloha or 

beta 
detector 



Signal- 
processing 
electronics 



Controlled 
air 
flow 



Detector 

bias 

supply 





Counter 
timer 








1 






Delay 
timer 


-* 


TTY 
scanner 


— 


Chronometer 






^' 








Printer 

and tape 

punch 





FIGURE 3. - Block diagram of CWL monitoring system. 



Figure 4 shows an unassembled model FC 
CWL detector. Figure 5 shows an assem- 
bled model FC detector mounted on a metal 
box that contains the signal-processing 
electronics (same as those used with the 
B-lOO detector). Collection air passes 
through the filter, which faces the de- 
tector, and then to the back side of the 
detector housing by means of a U-shaped 
piece of plastic tubing. 

The model FC CWL detector can also be 
used with an alpha detector by replacing 
the GM tube with a SOO-mm^ surface bar- 
rier diode. An appropriate preampli- 
fier is used along with the necessary 
detector bias supply. Figure 6 shows 
this arrangement. 

The beta detector used in the model FC 
CWL detector is a type-7242 end-window GM 
tube. It has a diameter of 2.54 cm and a 
thickness of 1.33 cm. It operates at an 
anode voltage of 750 V and has a mica 
window with an areal density of 2 mg/cm^. 
An aluminum foil is cemented over the GM- 
tube opening in the housing to prevent 
alpha measurements and to protect the GM 
tube from moisture. 

The model FC detector uses a 25-mm-diam 
filter, which tends to clog sooner than 
the 47-mm-diam filters used in the model 
B-lOO unit. The filter-to-detector dis- 
tance can be changed in the model FC, 
The most frequently used distance is 
about 1 cm. At this distance, unwanted 
airborne beta activity can penetrate to 
the detector. This can be prevented by 
using a shield as shown in figure 7. 

Measuring beta activity on the col- 
lection side of the filter results in 




FIGURE 4. - Unassembled model FC CWL detector. From left to right: back cover plate with 
signal and air connectors, end-window GM tube, plastic mounting ring for GM \u\)e, housing for 
CM tube, spacers, filterring, 0-ringfilter gasket, glass-fiber filter, and filter holderwith backup 
screen and tubing connector. 



5 



/ 




FIGURE 5. - Model FC CWL detector mounted 
on electronics box. 



greater sensitivity using the model FC , 
as compared with the model B-100. At a 
given airflow rate, the model FC is about 
2,7 times more sensitive to radon daugh- 
ters. This results, in part, from a 
greater efficiency in measuring the soft- 
er 214ptj (RaB) beta particles. Typical 
efficiencies are 11.7 pet for RaB and 
13.2 pet for RaC. For the model B-100, 
typical efficiences are 3.5 pet for RaB 
and 10.6 pet for RaC. A method for mea- 
suring these efficiencies is given in the 
appendix. 

Filters used for collecting radon 
daughters should have a high efficiency 
for collecting particles smaller than 
1 pun; this applies to both membrane- and 
glass-fiber-type filters. The membrane 
filter is a surface collector, while the 
glass-fiber filter is a depth collector. 
This characteristic of the glass-fiber 
filter makes it prone to self-absorption 
for alpha and beta particles. Beta- 
absorption values for five 47-mm-diam 
glass-fiber filters, using a 210b , beta 
source located on the opposite side of 
the filter from a GM detector, were as 
follows, in percent: 17.5, 20.0, 17.6, 
17.7, and 17.5. 




FIGURE 6. - Model FC CWL detector with alpha-detecting diode. The metal enclosure contains a 
preamplifier. 



It is important to keep the filter-to- 
detector distance constant to maintain 
calibration in all CWL measuring systems. 
Table 1 shows the effect on count rate of 
changing the source-to-detector distance, 
using a 210gj beta source and a beta de- 
tector such as the type used in the model 
B-100. At the filter-to-detector dis- 
tance used in the B-100, a 1-mm change in 
distance results in a change in the cali- 
bration factor of several percent. The 
filter must be kept in place against the 
filter backup screen. It will usually 
stay in place while air is applied to the 
CWL detector. The calibration of alpha 



TABLE 1. - Effect of filter-to-detector 
distance on count rate 

(Using 210b i beta source, glass-fiber 
filter, and aluminum backup screen) 



Distance, mm 
9.39 

10.89 

12.89 

15.89 

18.89 

21.89 

26.89 



Count , cpm 



2,913 
2,736 
2,573 
2,314 
2,100 
1,861 
1,553 



Std dev, cpm 



±27.3 
±26.5 
±25.7 
±24.4 
±23.3 
±21.9 
±20.1 




FIGURE 7. - Model FC CWL detector with beta shield. 



and beta detectors is affected by changes 
in the air-path density between the de- 
tector and the source. 

AMBIENT AIR COUNTING 

Several years ago, the Bureau began 
evaluating a method of measuring working- 
level exposures by measuring the beta 
activity of the air surrounding a beta- 
detecting system. Figure 8 shows one of 
the prototype detectors developed by the 
Bureau and tested in the Bureau's exper- 
imental uranium mine. This detector, 
called the model Beta-8, consists of two 



counting sections. The lower section has 
four GM tubes operating in parallel with 
no shielding. These tubes can be seen at 
the lower end of the detector in figure 
8. The upper section also has four GM 
tubes of the same type as the lower sec- 
tion, but these tubes are shielded 
against the ambient beta activity, re- 
sulting in gamma ray activity measure- 
ments only for this section. By sub- 
tracting the count from the upper section 
from the count from lower section, a 
fairly good approximation of the ambient 
beta activity can be made without the use 
of a filter and air system. 




Air exhaust 



FIGURE 8. - Model Beta-8 experimental CWL 
detector. 



Airflow 
meter 



Air pump 



Airflow 
controller 



Differential 
pressure 



Airflow 
sensor 



Electronic 
manometer 




To CWL detector To data- 

and air input aquisition system 

FIGURE 9. - Block diagram of typical air sys- 
tem for use with a CWL detector. 



Mine tests of the Beta-8 detector 
showed that it has a rapid response time 
to changes in working level when the con- 
densation nuclei level exceeds about 
25,000 particles per cubic centimeter. 



At lower concentrations, plateout of ra- 
don daughters on the detector causes 
response and accuracy problems. At pres- 
ent, no practical method exists for pre- 
venting the radon daughter plateout. 



AIR SYSTEMS 



All methods for measuring radon daugh- 
ters collected on a filter paper require 
a known air volume or volumetric flow 
rate. In most of its work with CWL de- 
tectors, the Bureau uses instrumentation 
to measure flow rate continuously. In 
addition, a mechanical flow controller is 
used to set and hold the flow rate at a 
fixed value over a wide range of differ- 
ential pressures across the filter. Fig- 
ure 9 shows a block diagram of a typical 
air system. 

Continuous airflow rate measurements 
are made either by mass flow sensors and 
converted to volumetric flow rate or by 
direct measurement with volumetric flow 
sensors using a laminar flow rate. Flow 



rates are also checked with a dry test 
meter connected in an appropriate part of 
an air system. 

Most of the air systems used in the Bu- 
reau's studies of CWL detectors include a 
pressure gauge to measure the differen- 
tial pressure across the collection fil- 
ter. The gauge serves two purposes: 

(1) It indicates the presence of a leak 
around a newly installed filter, and 

(2) it indicates when the filter should 
be changed because of clogging. 

The sensitivity of a CWL detector is 
directly proportional to the airflow 
rate. In uranium mine applications, a 
flow rate of 1 to 2 L/min provides good 
sensitivity. For sites where the radon 



daughter activity is low, such as in 
dwellings, sampling flow rates can be 



increased to improve the statistical ac- 
curacy of the measured count. 



FACTORS AFFECTING ACCUEIACY 



There are a number of factors that can 
affect the accuracy of a working-level 
measurement made by CWL detectors. Many 
of these factors apply to any measurement 
of radioactivity and have been discussed 
in the literature. Several of the more 
important factors that affect the accu- 
racy CWL detectors of the type covered in 
this report are discussed below. 

VOLUMETRIC AIRFLOW VARIATIONS 

Volumetric airflow variations are an 
obvious problem, but one that is not eas- 
ily resolved. Various approaches to flow 
control have been used, including differ- 
ential flow controllers, electromechan- 
ical systems that control air-pump 
speeds, and the use of either mass or 
volumetric airflow rate sensors, which 
usually require monitoring by the data- 
acquisition system. Airflow rate varia- 
tions are a common source of error for 
CWL systems. 



and 214pq) is represented by a line that 
originates at the midpoint of the hypote- 
nuse and bisects the triangle. The line 
showing no inherent uncertainty passes 
through the mixture of RaA, RaB, and RaC 
upon which the calibration is based. 

Figure 10 shows the inherent uncertain- 
ty triangle for a CWL detector measuring 
alpha particles. In this case, the error 
is quite low and can be ignored for most 
measurements. The error direction is to- 
ward overestimation of the working level 
as the radon daughter mixture becomes 
dominated by RaA. This provides some er- 
ror compensation for plateout losses as- 
sociated with radon daughter mixtures 
high in RaA and environments with rela- 
tively low concentrations of condensation 
nuclei. 

The inherent uncertainty triangle for a 
model B-lOO CWL detector measuring beta 
activity is shown in figure 11. Differ- 
ences between the radon daughter mix- 
ture and the calibration mixture have a 



RADON DAUGHTER MIXTURE VARIATIONS 

When airborne radon daughter mixtures 
differ from the mixture used for cali- 
brating a gross-count CWL detector, the 
working-level measurement will have an 
error that is called inherent uncertain- 
ty. This error originates from assigning 
an average energy to each count measured 
when making the calibration. The degree 
of inherent uncertainty depends on how 
much the average energy per count dif- 
fers from the calibration mixture and 
whether alpha or beta particles are being 
measured. 

The degree of inherent uncertainty can 
be shown by a triangular graph ( 16-17 , 
19) . Briefly, the graph represents all 
possible mixtures of RaA, (218 Po)» RaB, 
and RaC equivalent to 1 WL. One leg of 
the triangle represents RaA activity in 
picocuries per liter per working level, 
and the other leg represents the RaB ac- 
tivity in the same units. RaC-C (214qj 



RaB, pCi/(L/WL) 
50 100 150 




FIGURE 10. - Inherent uncertainty plot for a gross 
alpha CWL detector, showing percent inherent uncer- 
tainty. fPlus sign indicates equilibrium mixture.) 



10 



RaB, pCi/(L/WL) 
50 100 150 




RaB, pCi/(L/WL) 
50 100 



50 



FIGURE 11. - Inherent uncertainty plot for a gross 
beta CWL detector counting from back side of filter, 
showing percent inherent uncertainty. (Plus sign in- 
dicates equilibrium mixture.) 




FIGURE 12. - Inherent uncertainty plot for a gross 
beta CWL detector counting from collection side of 
filter, showing percent inherent uncertainty. (Plus 
sign indicates equilibrium mixture.) 



greater effect on this method of mea- 
suring working-level exposures. This is 
also true for the model FC CWL detector, 
whose inherent uncertainty triangle is 
shown in figure 12. In both these 
graphs, the measured efficiencies for 
beta activity from RaB and RaC were used 
to construct the error lines. For gross 
beta measurements, the inherent uncer- 
tainty error increases as the radon 
daughter mixture becomes dominated by 
RaA. In most applications, this type of 
error seldom exceeds 8 pet. 

PLATEOUT LOSSES 

CWL detector errors caused by plateout 
losses are not easy to assess. Errors 
from plateout become noticeable when the 
condensation nuclei concentration drops 
below 10,000 particles per cubic centime- 
ter and may be quite significant below 
5,000 particles per cubic centimeter. 
Plateout losses are a greater problem 
with detector systems designed to count 
the collection side of the filter; they 
do not pose a significant problem for 
beta detectors that measure from the back 
side of the filter. 



OTHER SOURCES OF ERROR 

Other sources of error previously men- 
tioned include self-absorption by the 
filter and changes in the distance and 
air density between the collecting sur- 
face of the filter and the detector. 

The use of depth filters, such as those 
made of glass fiber, results in some loss 
due to self-absorption. However, most of 
this loss is compensated for when the 
detector is calibrated. Surface filters, 
such as membrane filters, present little 
or no self -absorption problems with most 
aerosols when measurements are from the 
collection side of the filter. However, 
surface filters tend to clog much sooner 
than depth filters, making them less 
suitable for use in environments with 
relatively high smoke levels. Self- 
absorption problems encountered when 
counting from the back side of a filter 
are similar for both surface and depth 
filters. 

Tests on the Bureau's model B-lOO CWL 
detector show that there is a significant 
change in the count rate when the dis- 
tance between the source and the beta 
detector is changed. (This effect is 



11 



discussed in the previous section, "Fil- 
ter Surface Counting.") 

Dust and diesel smoke can affect the 
detector's response by increasing absorp- 
tion of alpha and beta activity and by 
changing source-to-detector distances. 
This problem can be prevented by timely 
filter replacements. 

In uranium mines, where CWL detectors 
are operated during periods when the 



ventilation system is off, significant 
amounts of 210g j activity will build up 
on the filter, causing an error in the 
measurements. This problem can be pre- 
vented by weekly filter changes. 

Thoron daughters can also cause errors, 
but this has not been a problem in domes- 
tic mines. 



CALIBRATION 



CWL detectors are usually calibrated by 
operating them in an atmosphere where the 
radon daughter activity can be accurately 
determined. The best place to do this is 
in a properly designed radon test cham- 
ber, where the factors that affect cali- 
bration can be controlled reasonably 
well. Calibration can also be done on- 
site, such as in a mine, but a mine envi- 
ronment has constantly changing condi- 
tions that make calibration based on grab 
sampling difficult. This problem can be 
overcome by using a calibrated CWL detec- 
tor to calibrate the unknown unit. 

Prior to calibration, a system using a 
GM tube must be adjusted for operation at 
the midpoint of the detector plateau, and 
the system dead time must be known. For 
systems that use an alpha detector, a 
lower level discriminator must be set to 
a point where beta activity, as well as 
the detector noise, is not measured. 

The first step in calibrating a system 
that uses a GM detector is to measure the 
background radiation (primarily gamma ac- 
tivity) at the operating site of the CWL 
detector. The Bureau uses a separate 
background detector operated near the CWL 
detector for this purpose. The gamma ray 
response between the two detectors is de- 
termined and used to correct the count 
from the CWL detector on a continuous ba- 
sis. Alpha detectors should have in- 
significant background levels in most 
applications. 

Before applying air to the CWL detector 
using a GM tube, the detector should be 
standardized with a gamma ray source, 
such as 137(^5. The source should be 
placed at a fixed distance from the de- 
tector and the detector's response re- 
corded for future reference. For systems 



using alpha detectors, a 230-ph source on 
a disk that replaces the filter paper can 
be used for standardization. 

Following the standardization proce- 
dure, air at a known flow rate is applied 
to the CWL detector. After a period of 
4 h or more in a controlled radon daugh- 
ter atmosphere , the count rate is mea- 
sured and corrected for dead-time losses 
and background. Modified Tsivoglou (21) 
or alpha spectroscopic methods are used 
to measure the levels of radon daughter 
activity. This information is converted 
to working-level values , and a count con- 
version factor is determined by the 
equation 



Kp = (WL/Na)/F, 



(1) 



where 



and 



Kp = count-to-working-level con- 
version factor, 

Na = average counts per minute 
corrected for dead time and 
background, cpm, 

WL = average working level for 
calibration period, 

F = calibration flow rate, 
L/min. 



Correction for the system dead time can 
be made by using the equation 



N = 



1-nt 



(2) 



where N = true count rate, cpm, 

n = observed count rate, cpm. 



12 



and 



t = dead time, tain. 



The dead time for the counting system can 
be measured using an oscilloscope. 



Another method for determining the 
count-to-working-level conversion factor 
for beta detectors is given in the 
appendix. 



ATTACHMENTS FOR MODEL B-lOO CWL DETECTOR 



The unobstructed filter-collection in- 
put of the model B-lOO CWL detector fa- 
cilitates attaching devices to modify the 
detector's response. One such device is 
shown in figure 13. Input tubes have 
been mounted on the filter clamp rings of 
two model B-lOO units. One of the tubes 
has a 60-mesh wire screen (13) at its en- 
trance. Figure 14 shows the effects of 
this screen relative to the amount of 
condensation nuclei in a test chamber at- 
mosphere. The measurements were made 
with the chamber mixing fans off and with 
condensation nuclei generated by a nebu- 
lizer using tap water. The ratio of the 
working-level values measured by the two 
B-lOO detectors (designated B-102 and 
B-101) reflects the amounts of attached 
and unattached radon daughter particles 
in the test chamber atmosphere. 

Figure 15 represents similar measure- 
ments made in the Bureau's Twilight 




FIGURE 13. - Two model B-lOO CWL detectors 
equipped with input tubes. Detector on left has a 
60-mesh screen at the tube input. 



experimental uranium mine over a period 
of several days. The results show the 
effect of diesel smoke on the ratio of 
attached-to-unattached radon daughters. 

Figure 16 shows two Zeleny tubes 
mounted on the filter clamp rings of two 
model B-lOO detectors. The tubes are 
identical except for a charged wire along 
the axis of one of them (fig. 17). These 




FIGURE 14. - Effect of input screen on attached- 
unattached radon daughter activity, with changes ir 
concentration of condensation nuclei. 




No diesel smoke 



Diesel smoke 



55 56 



57 



58 



59 60 61 62 
DAY OF YEAR 



53 



64 



65 66 



FIGURE 15. - Effect of input screen on attached- 
unattached radon daughter activity in the Twilight 
Mine. 



13 




FIGURE 16. - Two model B-lOO CWL detectors 
equipped with Zeleny tubes. Detector on right con- 
tains a charged wire. 

tubes are patterned after work by Busigin 
(J^) . Figure 18 shows the results of 
operating with negative 1,000 V on 
the wire of detector B-102 and changing 




FIGURE 17. - Model B-lOO CWL detector with 
Zeleny tube containing a charged wire. 



the amount of condensation nuclei in a 
radon test chamber under the same condi- 
tions as indicated above (for the screen 
measurements) . 



CWL DETECTOR APPLICATIONS 



Commercial CWL monitors have been used 
primarily to monitor dwellings and radio- 
active waste disposal sites. Their use 
in mining has been very limited (14-15, 
18). 

The Bureau has used CWL monitors in its 
radiation hazards research program for 
over 8 yr (,3_, J_l_) . CWL monitors have 
been especially useful in calibrating ra- 
don daughter dosimeters and in studies of 
radon daughter levels in various mine 
atmospheres. Franklin (J^) describes an 
alarm system designed to warn of working- 
level exposures that exceed a preset val- 
ue. Such a system is useful in providing 
mine operators with timely information on 
radiation levels at work sites and in 
main airways. 



The Bureau's model B-lOO CWL detector 
has been used to investigate the effi- 
ciency of a ventilating system equipped 
with a heat exchanger in removing radon 
daughters from the basement of a dwell- 
ing. Figure 19 shows the working-level 
measurements for a period of 2 weeks. 
During this time, air exchanges between 
the basement and the outside were nor- 
mal except during the period marked on 
the graph when the heat-exchanger system 
was operating at 0.5 air exchanges per 
hour. The graph shows that the system 
was effective in lowering the working- 
level exposure to acceptable values for a 
portion of the house (where the bedrooms 
were) . 



14 



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TIME.h 

FIGURE 18. - Charged particle collection using a negatively charged wire in a Zeleny tube 
a model B-lOO CWL detector. A positive charge on the wire also results in the collection of 
don daughters. 



< 
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33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 

DAY 

FIGURE 19. - Effect of a ventilation system using a heat exchanger on the working-level ex- 
posure in a dwelling. 



15 



COMMERCIAL CWL INSTRUMENTS 



Figures 20 and 21 show two C\^ monitor- 
ing systems that are available from com- 
mercial sources (]_~^) ' Both of these 
systems measure alpha activity and use a 
membrane filter. Figure 22 shows a com- 
parison of measurements made by a commer- 
cial (alpha) system and by the Bureau's 
model B-lOO (beta) detector during the 
monitoring of a basement in a dwelling. 
The commercial system was operated with 
60-min readout intervals, whereas the Bu- 
reau's system measured for 60 min and 



then delayed for 40 min. There is no 
significant difference between the re- 
sults obtained using the two systems. 

Figure 23 shows a commercial version of 
the Bureau's model B-lOO CWL detector 
with the access doors open. The unit was 
built under a Bureau contract (20) . Un- 
like the two commercial systems mentioned 
above, the system shown in figure 23 does 
not include a data-acquisition portion, 
which must be supplied by the user. 



CONCLUSIONS 



CWL detectors have been widely used in 
the Bureau's radiation-hazard research 
program for the past several years. 
Their application has been very useful 
in long-term calibration of personal do- 
simeters, working-level alarm systems 
for mines, and the study of long-term 




relationships between radon and radon 
daughters in mine atmospheres. Future 
applications are expected to include the 
use of these detectors in characterizing 
radon daughters and their relationship to 
aerosols found in mine environments. 




FIGURE 20. - A commercial CWL monitoring 
system in operation in a mine. 



FIGURE 21. - A commercial CWL monitoring 
system undergoing tests in a Bureau laboratory. 



16 








18 19 20 21 



22 23 24 25 
DAY OF YEAR 



26 27 28 29 



FIGURE 22. - Comparison of working-level values measured by alpha and beta CWL detec- 
tors in a dwelling. 



17 




FIGURE 23. ■ Commercial version of model B-lOO CWL detector (mounted at bottom of box at 
left)- Metal box on right contains an oir system for use with the CWL detector. 

REFERENCES 



1. Busigin, C. J., A. Busigin, and 
C. R. Phillips. Measurement of Charged 
and Unattached Fractions of Radon and 
Thoron Daughters in Two Canadian Ura- 
nium Mines. Health Phys., v. 44, 1983, 
pp. 165-167. 

2. Droullard, R. F. Instrumentation 
for Measuring Uranium Miner Exposure to 
Radon Daughters. Paper in Radiation Haz- 
ards in Mining: Control, Measurement, 
and Medical Aspects, ed. by M. Gomez 
(Proc. Int. Conf., Golden, CO, Oct. 4-9, 
1981). Soc. Min. Eng. AIME, Littleton, 
CO, 1981, pp. 332-338. 

3. E>roullard, R. F., T. H. Davis, 
E. E. Smith, and R. F. Holub. Radiation 



Hazard Test Facilities at the Denver Re- 
search Center. BuMines IC 8965, 1984, 
22 pp. 

4. Droullard, R. F., and R. F. Holub. 
Continuous Working Level Measurements 
Using Alpha or Beta Detectors. BuMines 
RI 8237, 1977, 14 pp. 

5. . A Continuous Working Level 
Monitor^ HASL No. 325, ERDA Radon Work- 
shop, New York, Feb. 1977, pp. 43-47. 

6. Droullard, R. F., and R. F. Holub. 
Method of Continuously Determining Radia- 
tion Working Level Exposures. U.S. Pat. 
4,185,199, Jan. 22, 1980. 



18 



7. Eberline Instrument Corp. (Santa 
Fe, NM). WLM-l/WLK-1 Radon Working Level 
Monitoring System. 1982, 20 pp. 

8. EDA Instruments, Inc. (Toronto, 
Canada) . Operating Manual for WLM-300. 
1981, 14 pp. 

9. Evans, R. D. Engineer's Guide 
to the Elementary Behavior of Radon 
Daughters. Health Phys . , v. 17, 1969, 
pp. 229-252. 

10. Franklin, J. C, P. E. Barr, K. D. 
Weverstad, and C. T. Sheeran. Alarm 
System for Radiation Working Level, Fan 
Operation, and Air Door Position. Bu- 
Mines IC 8903, 1982, 17 pp. 

11. Franklin, J. C, and R. F. Droul- 
lard. Instrumentation Developed by the 
Bureau of Mines for Continuously Monitor- 
ing Radon and Radon Daughters. ISA 
Trans., v. 22, No. 4, 1983, pp. 25-32. 

12. Fu-Chia, Y. , and T. Chia-Yang. A 
General Formula for the Measurement of 
Concentrations of Radon and Thoron Daugh- 
ters in Air. Health Phys., v. 34, 1978, 
pp. 501-503. 

13. George, A. C. Measurement of the 
Uncombined Fraction of Radon Daughters 
With Wire Screens. Health Phys., v. 23, 
1972, pp. 390-392. 

14. Haider, B., and W. Jacobi. Long- 
term Measurements of Radon Daughter Ac- 
tivity in Mines. Paper in Proceedings 
of the Third International Congress of 
the International Radiation Protection 
Association (Washington, DC, Sept. 9-14, 



1973). U.S. AEC CONF-730907-P2, 1973, 
pp. 920-925. 

15. Holmgren, R. M. Working Levels of 
Radon Daughters in Air Determined From 
Measurements of RaA and RaC, Health 
Phys., V. 27, 1974, pp. 141-145. 

16. Holub, R. F. Evaluation and Mod- 
ification of Working Level Measurement 
Methods. Health Phys., v. 39, 1980, 
pp. 425-447. 

17. Holub, R. F., and R. F. Droullard. 
Evaluation of Various Radon Daughter Mea- 
surement Methods. Paper in Workshop on 
Methods of Measuring Radiation In and 
Around Uranium Mills, ed. by E. D. Har- 
vard (Albuquerque, NM, May 23-26, 1977). 
Atomic Industrial Forum, Inc., Washing- 
ton, DC, 1977, pp. 197-219. 

18. Kawaji, M. , H, Lin Pai , and C. R. 
Phillips, Use of Gross Filter Activities 
in a Continuous Working Level Monitor. 
Health Phys., v. 40, 1981, pp. 543-548. 

19. Schiager, K. J., T. B. Borak, and 
J. A. Johnson (Alara, Inc.). Radiation 
Monitoring for Uranium Miners: Evalua- 
tion and Optimization (contract J0295026, 
Alara, Inc.). BuMines OFR 149-82, 1981, 
132 pp.; NTIS PB 83-102681. 

20. Strombotne, T. R. , and A. L. 
Beggs. Continuous Working Level Detector 
System (contract H0212005, TSA Systems, 
Inc.). BuMines OFR 2-83, 1982, 21 pp. 

21. Thomas, J. W. Measurement of Ra- 
don Daughters in Air. Health Phys., v. 
23, 1972, pp. 783-789. 



19 



APPENDIX. —METHOD FOR MEASURING EFFICIENCY OF A BETA DETECTOR 



The efficiency of a beta detector for 
measuring 214p and 214g. beta activity- 
can be determined by the method described 
in this section. 

The following equations are used: 



and 



and 
where 



I, = Eg Nb, + Ec Nc, 



I2 = Eb Nb2 + Ec Nc^, 



(A-1) 
(A-2) 



I] = count measured in first 
counting period. 



I2 = count measured in second 
counting period, 



Eg and Eq 

Nb,. Nc 

Nb2> a"d Nc2 



and Nb,, Nc,, 



unknown efficiencies for 
RaB and RaC , 

nuiuber of atoms that de- 
cay on the filter paper 
during the two measure- 
ment periods. 



The number of atoms (Ng , etc.) must be 
determined independently by measuring the 
radon daughter activity, using either the 
Tsivoglou (^) or alpha spectroscopic 
method. 

Solving the linear simultaneous equa- 
tions A-1 and A-2 for Eg and Eq provides 
the desired information. Repeated exper- 
iments show a reproducibility within ±10 
pet of the individual efficiencies, re- 
flecting the counting statistics in both 
I) and I2 and in the determination 
of Ng, , etc. 

The accuracy of the beta efficiencies. 
Eg and Eq, can be checked by comparing 
their weighted sum to the steady-state 
equilibrium efficiency of a continuously 
operating beta detector using the follow- 
ing equation: 



fc = 



Ng + Nc 



Both of these values are determined at 
the equilibrium at which the steady-state 
efficiency was determined. 

A typical procedure for measuring the 
beta efficiencies is outlined below, 

1. Measure RaA, RaB, and RaC using the 
modified Tsivoglou method. 

2. Measure the background of the beta 
detector with no activity on the collec- 
tion filter. 

3. Collect a 15-min sample at a known 
flow rate. 

4. Count the beta activity on the fil- 
ter for 10 rain, starting -2 min after the 
end of collection (2 min to 12 min). 
Make dead-time corrections and normalize 
to 1.0 L/min. Subtract background. The 
resulting difference is I^. 

5. Count the beta activity on the fil- 
ter for 10 min, starting 17 min after the 
end of collection (17 min to 27 min). 
Make dead-time corrections and normalize 
to 1.0 L/min. Subtract background. This 
difference is I2. 

6. After completing the collection us- 
ing the beta detector, measure the RaA, 
RaB, and RaC activity again. 

7. Determine the mean RaA, RaB, and 
RaC values. 

8. Calculate the Ng and Nq values for 
the 2-12 and 17-27 time intervals (steps 
4 and 5), using equations from reference 
12. These values become Ng and Nc„ for 
the 2-12 interval and Ng and Nc for the 
17-27 interval. 

9. Solving equations A-1 and A-2, find 
the determinant 

DET = Ng, X Nc, - N2 X Nc,, 



^B, TOTAL - fe >< Eg + fc X E( 



(A-3) 



then Eg = 



I, X Nr_ - lo X Nf 



DET 



(A-5) 



where 



fR = 



Nh + Nf 



and 



Ec = 



- I 



DET 



(A-6) 



20 



An example of this method for determin- 
ing beta efficiencies for the model FC 
(beta) CWL detector is given below. 

Step 1 

RaA = 743.05 pCi/L 

RaB = 303.17 pCi/L 

RaC = 167.32 pCi/L 

Exposure = 2.942 WL 

Step 2 

Background = 71.8 cpm 

Steps 3-6 

Flow rate = 1.893 L/min 

Dead time = 47 |is 

Gross count (2-12) = 38,720 

Gross count (17-27) = 32,894 

Net count (2-12) = 38,120 

Net count (17-27) = 32,261 

Count normalized to 1 L/mln (2-12) 

= 20,137 

Count normalized to 1 L/min (17-27) 

= 17,042 

Steps 7-8 

Nb,(2-12) = 90,139 dis/min at 1 L/min 

Nc,(2-12) = 72,558 dis/min at 1 L/min 

Nb2( 17-27) = 62,700 dis/min at 1 L/min 



Nc2( 17-27) = 73,477 dis/min at 1 L/min 

Step 9 

DET = (90,139)(73,477) - (62,700)(72,558) 

„ _ ( 20,137)(73,477) - (17,042) (72,558 ) 
^B - DET 

= 0.1172 (11.72 pet) 

( 17,042)(90,139) - (20,137)(62,700 ) 
C DET 

= 0.1319 (13.19 pet) 

Using the above values of RaA, RaB, and 
RaC (from step 1) and normalizing them to 
1 WL and 1 L/min, the equilibrium filter 
activity would be as follows: 

Nr = 11,399 dis/min, 

eq 

Nr = 14,987 dis/min, 

eq 

and No + Np = 26,386 dis/min, 

°eq ^ eq 

Using equation A-3, the total beta 
count can be calculated as follows: 

Eb, total = (0.432)(0.1172) 

+ (0.568)(0.1319) 

= 0.1255 (12.55 pet). 

In this example, 12.55 pet of 26,326 dis/ 
min gives a count rate of 3,313 cpm, and 
the reciprocal of this number gives a 
eount-to-working-level conversion factor 
of 0.000302. The conversion factor de- 
termined by the usual calibration method 
described in the main text was 0.000307, 
which is in good agreement with the val- 
ue determined using the detector beta 
efficiencies. 



irU.S. GPO: 1985-505-019/20,066 



INT.-BU.OF MINES, PGH., PA. 28014 



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